N-aminoethylpiperazine (“AEP”) is widely used in conjunction with polyoxyalkylene polyamines, e.g. JEFFAMINE®D-230 amine and JEFFAMINE®T-403 amine, to serve as an accelerator for increasing the polymerization rate of epoxy resins cured with such hardeners. AEP reacts with most epoxy resins much more quickly than other amine curing agents, especially at moderate ambient temperatures (e.g. “room temperature”). Among the reasons believed for this is the presence of a tertiary amine in the AEP molecule and the relatively low amine hydrogen equivalent weight (AHEW) of the compound. Having a relatively low AHEW means that the concentration of reactive groups in a given formulation will be increased relative to a higher AHEW amine hardener or amine hardener blend.
Shortages of AEP have created a need for a substitute material that can serve to accelerate the curing reaction of slower amine curatives (e.g. JEFFAMINE® brand amines). Although other accelerators of amine cured epoxy blends exist, they each have particular drawbacks that can make them unsuitable for certain applications. For instance, phenolic accelerators are often solids and contribute undesired color or ultraviolet light sensitivity to the final formulation. Widely used liquid accelerators, e.g. nonyl phenol, mono-nonyl phenol (MNP) etc., also serve as plasticizers, significantly and undesirably lower the glass transition temperatures (Tg) of resin systems into which they are incorporated at levels high enough to provide significant acceleration. Additionally, the accelerating effect diminishes as increasing levels of MNP are used since the reactant group concentrations are diminishing. Tertiary amines containing high levels of hydroxyl groups, such as triethanolamine, methyldiethanolamine, dimethylethanolamine, etc., have been effectively used as accelerators but since they remain as small molecules that do not react into the polymer network, they too are known to cause significant decreases in Tg.
In practice, many customers use AEP at less than about twenty weight percent to shorten the gel time of epoxy formulations that contain JEFFAMINE® brand amines as hardeners. AEP is a somewhat unusual amine in that it contains a primary, a secondary, and a tertiary amine. It has provided some of the highest exotherm temperatures seen when used to cure epoxy resins. Providing high exotherm temperatures can be advantageous to promote increased curing but can lead to polymer degradation if unchecked. If used as the sole hardener to cure diglycidylether of bisphenol A (“DGEBA”)-type epoxy resins, AEP provides a Tg higher than that obtained by curing the resin with JEFFAMINE®D-230 amine. Thus, the use of AEP as an accelerator will not decrease the glass transition temperatures when used at a 1:1 stoichiometry in the formulation.
The formation of epoxy polymers from polyepoxies and polyamines is well-known in the art. These crosslinked polymer networks are formed from the reaction between a polyepoxy such as: a polyamine, which may be a diamine:H2N—R—NH2such crosslinked polymers with segments similar to: are well-known in the art, and are the reaction products of various epoxy containing compounds and various polyamines and polymers produced from the combination thereof.